Interpretive Summary: Infection of the udder (mastitis) is a major economic problem for the dairy industry. In the dairy industry, this disease alone accounts for 2 billion dollars a year in losses to the industry. Bacteria, such as Escherichia coli (E. coli), are the major cause of mastitis in dairy cows. Some bacteria cause a short duration infection and others cause a persistent infection. We compare 3 E. coli strains that cause a transient infection with 3 E. coli strains that cause a persistent infection. Proteomics is the identification of proteins made in a cell. Previous protein identification techniques did so one at a time where proteomics can identify hundreds of proteins in a sample. We have identified over 1000 proteins in E. coli, and narrowed them down to 28 that have significant expression differences when comparing the E. coli strains that cause a transient versus a persistent infection. Many of these proteins are important in how the bacteria move. We tested bacteria motility and found that certain motility functions are better in persistent strains of E. coli than in transient strains. This will help us better identify and understand persistent infections and this will lead to better diagnostics and therapeutics.

Technical Abstract:
Escherichia coli is a leading cause of bacterial mastitis in dairy cattle. Typically this infection is transient in nature, causing an infection that lasts 2-3 days. However, in a minority of cases, E. coli has been shown to cause a persistent intramammary infection. The mechanisms that allow for a persistent E. coli infection are not fully understood. The goal of this work was to determine protein expression differences between E. coli strains isolated from dairy cattle with transient or persistent mastitis infections. Three persistent and three transient mastitis-derived strains of E. coli were compared using multiplexed amine-modifying labeling reagents (iTRAQ) in a shotgun proteomics experiment. Expression data for 1127 proteins were determined. Of these, 27 E. coli proteins were associated with expression changes correlated with a difference in disease phenotype. Of particular interest are the Tsr and Tar proteins, which have been shown to be essential for bacterial swimming and swarming. Both Tsr and Tar were shown to be up-regulated in E. coli strains that caused persistent infection compared to the transient strains. Bacterial swimming and swarming assays showed that the strains from the persistent mastitis cases were significantly better in these motility assays than the strains from the transient cases. Proficient swimming and swarming are thought to be a component of bacterial virulence. This work identifies important protein expression differences between E. coli strains that cause a persistent versus a transient infection as well as demonstrates a corresponding difference in the associated bacterial motility phenotypes.